Digital lighting technologies, i.e. illumination based on semiconductor light sources, such as LEDs, offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g. red, green, and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Pat. Nos. 6,016,038 and 6,211,626, incorporated herein by reference.
A DC-DC converter is a well-known electrical device that accepts a DC input voltage and provides a DC output voltage to a load. DC-DC converters generally are configured to provide a regulated DC output voltage or current to a load (a “load voltage” or “load current”) based on an unregulated DC source voltage which in some cases is different from the output voltage. For example, in many automotive applications in which a battery provides a DC power source having an unregulated voltage of approximately 12 Volts, a DC-DC converter may be employed to receive the unregulated 12 Volts DC as a source and provide a regulated DC output voltage or current to drive various electronic circuitry in a vehicle (instrumentation, accessories, engine control, lighting, radio/stereo, etc.). The DC output voltage may be lower, higher or the same as the source voltage from the battery.
More generally, a DC-DC converter may be employed to transform an unregulated voltage provided by any of a variety of DC power sources such as batteries to a more appropriate regulated voltage or current for driving a given load. In some cases, the unregulated DC source voltage may be derived from an AC power source, such as a 120 Vrms/60 Hz AC line voltage which is rectified and filtered by a bridge rectifier/filter circuit arrangement. In this case, protective isolation components (e.g., a transformer) may be employed in the DC-DC converter to ensure safe operation, given the potentially dangerous voltages involved.
FIG. 1 illustrates a circuit diagram of a conventional step-down DC-DC converter 100 configured to provide a DC load voltage 102 (Vload) and a regulated load current 103 (Iload) to a load 104 based on a higher unregulated DC source voltage 112 (Vsource). In exemplary lighting applications, the load 104 may be a light source such as one or more LEDs. The unregulated source voltage Vsource is expected to vary slightly (and randomly) over some relatively small range around a nominal value; however, in conventional DC-DC converter configurations, the source voltage Vsource would not be intentionally varied. The step-down converter of FIG. 1 also is commonly referred to as a “buck” converter.
DC-DC converters, like the buck converter of FIG. 1, employ a transistor or equivalent device that is configured to operate as a saturated switch which selectively allows energy to be stored in an energy storage device (e.g., refer to the transistor switch 122 and the inductor 124 in FIG. 1). Although FIG. 1 illustrates such a transistor switch as a bipolar junction transistor (BJT), field effect transistors (FETs) also may be employed as switches in various DC-DC converter implementations. By virtue of employing such a transistor switch, DC-DC converters also are commonly referred to as “switching regulators” due to their general functionality.
In particular, the transistor switch 122 in the circuit of FIG. 1 is operated to periodically apply the unregulated DC source voltage 112 (Vsource) across an inductor 124 for relatively short time intervals (in FIG. 1 and the subsequent figures, unless otherwise indicated, a single inductor is depicted to schematically represent one or more actual inductors arranged in any of a variety of serial/parallel configurations to provide a desired inductance). During the intervals in which the transistor switch is “on” or closed (i.e., passing the source voltage Vsource to the inductor), current flows through the inductor based on the applied voltage and the inductor stores energy in its magnetic field. If the inductor current IL exceeds the load current Iload when the transistor switch is closed, energy is also stored in a filter capacitor 126. When the switch is turned “off” or opened (i.e., the DC source voltage is removed from the inductor), the energy stored in the inductor is transferred to the load 102 and the filter capacitor 126 which functions with the inductor 124 to provide a relatively smooth DC voltage Vload to the load 102 (i.e., when the inductor current IL is less than the load current Iload, the capacitor supplies the difference to provide essentially continuous energy to the load between inductor energy storage cycles). In continuous mode, not all of the energy stored in the inductor is transferred to either the load or the capacitor.
More specifically, in FIG. 1, when the transistor switch 122 is on, a voltage VL=Vload−Vsource is applied across the inductor 124. This applied voltage causes a linearly increasing current IL to flow through the inductor (and to the load and the capacitor) based on the relationship VL=L·dIL/dt. When the transistor switch 122 is turned off, the current IL through the inductor continues to flow in the same direction, with “freewheeling” diode 128 now conducting to complete the circuit. As long as current is flowing through the freewheeling diode 128, the voltage VL across the inductor is fixed at Vload−Vx, causing the inductor current IL to decrease linearly as energy is provided from the inductor's magnetic field to the capacitor and the load. FIG. 2 is a diagram illustrating various signal waveforms for the circuit of FIG. 1 during the switching operations described immediately above.
Conventional DC-DC converters may be configured to operate in different modes, commonly referred to as “continuous” mode and “discontinuous” mode. In continuous mode operation, the inductor current IL remains above zero during successive switching cycles of the transistor switch, whereas in discontinuous mode, the inductor current starts at zero at the beginning of a given switching cycle and returns to zero before the end of the switching cycle. To provide a somewhat simplified yet informative analysis of the circuit of FIG. 1, the discussion below considers continuous mode operation, and assumes for the moment that there are no voltage drops across the transistor switch when the switch is on (i.e., conducting) and that there is a negligible voltage drop across the freewheeling diode 128 while the diode is conducting current. With the foregoing in mind, the changes in inductor current over successive switching cycles may be examined with the aid of FIG. 3.
FIG. 3 is a graph on which is superimposed the voltage at the point VX shown in FIG. 1 (again, ignoring any voltage drop across the freewheeling diode 128) based on the operation of the transistor switch 122, and the current through the inductor IL for two consecutive switching cycles. In FIG. 3, the horizontal axis represents time t and a complete switching cycle is represented by the time period T, wherein the transistor switch “on” time is indicated as ton and the switch “off” time is indicated as toff (i.e., T=ton+toff).
For steady state operation, it should be appreciated that the inductor current IL at the start and end of a switching cycle is essentially the same, as can be observed in FIG. 3 by the indication Io. Accordingly, from the relation VL=L·dIL/dt, the change of current dIL over one switching cycle is zero, and may be given by:
      dI    L    =      0    =                  1        L            ⁢              (                                            ∫              0                              t                on                                      ⁢                                          (                                                      V                    source                                    -                                      V                    load                                                  )                            ⁢                              ⅆ                t                                              +                                    ∫                              t                on                            T                        ⁢                                          (                                  -                                      V                    load                                                  )                            ⁢                              ⅆ                t                                                    )            
which simplifies to
                              (                                    V              source                        -                          V              load                                )                ⁢                  t          on                    -                        (                      V            load                    )                ⁢                  (                      T            -                          t              on                                )                      =    0    or                              V          load                          V          source                    =                                    t            on                    T                =        D              ,  where D is defined as the “duty cycle” of the transistor switch, or the proportion of time per switching cycle that the switch is on and allowing energy to be stored in the inductor. From the foregoing, it can be seen that the ratio of the output voltage to the source voltage is proportional to D; namely, by varying the duty cycle D of the switch in the circuit of FIG. 1, the load voltage Vload may be varied with respect to the source voltage Vsource but cannot exceed the source voltage, as the maximum duty cycle D is 1.
In the apparatus 100, the load 104 may be one or more LEDs, and the intensity or brightness of radiation generated by the LED(s) is proportional to the average power delivered to the LED(s) over a given time period. Accordingly, one technique for varying the intensity of radiation generated by the LED(s) involves modulating the power delivered to the LED(s). Since power is defined as the amount of energy transferred in a given time period (i.e., P=dW/dt), the power P provided to the load may be expressed as
      P    =                            ⅆ          W                          ⅆ          t                    =                                                  1              2                        ⁢                                          L                ⁡                                  (                                      I                    P                                    )                                            2                                T                =                              1            2                    ⁢                                    L              ⁡                              (                                  I                  P                                )                                      2                    ⁢          f                      ,where f=1/T is the switching frequency of the transistor switch 128. From the foregoing, it may be appreciated that the power provided to the load 104 may be modulated by varying one or both of the switching frequency f and the peak inductor current IP, given the inductance L of the inductor 124, where the peak inductor current IP is determined by the duty cycle D of the transistor switch 122. It should be appreciated, however, that in practice, the relationship between frequency and LED brightness may not be linear as indicated by the above expression. Rather, as the switching frequency is increased, the average current to the LED(s) increases as the amount of ripple, or peak-to-peak excursion is reduced. However, as the average current approaches the peak value, the amount of ripple is small, and further increases in switching frequency may yield diminishing returns.
Hence, as mentioned earlier, the conventional buck converter of FIG. 1 is particularly configured to provide to the load 104 a voltage Vload that is lower than the source voltage Vsource. To ensure stability of the load voltage Vload or load current Iload as shown in FIG. 1, the buck converter employs a feedback control circuit 130 to control the operation of the transistor switch 122, thereby regulating the load voltage or the load current. Generally, power for various components of the feedback control circuit 130 may be derived from the DC source voltage Vsource or alternatively from another independent source of power.
While one or both of the load voltage and load current may be regulated, different types of loads may lend themselves more readily to either voltage regulation or current regulation. For exampling, considering LEDs as one exemplary load, in some applications it may be preferable to regulate load current rather than load voltage (e.g., due to different forward voltages for different types of LEDs, and/or different numbers and arrangements of LEDs constituting the load). Accordingly, primarily for purposes of illustration, the feedback control circuit shown in FIG. 1 is configured for current regulation of an exemplary LED-based load. It should be appreciated, however, that for any of the switching regulator circuits discussed herein, one or both of the load voltage and the load current may be regulated via the feedback control circuit 130 by deriving one or more appropriate values representative of the load voltage and/or the load current.
For example, in the feedback control circuit 130 of FIG. 1, the load current Iload may be sampled by placing a grounded resistor Rsample having a relatively small resistance in series with the load 104. A voltage Vsample measured across the resistor Rsample may be provided as an input to the feedback control circuit 130 representative of the load current (alternatively, the load voltage Vload rather than the load current Iload may be sampled by generating a voltage Vsample via a voltage divider (not shown) placed in parallel with the load 104). The sampled voltage Vsample may be compared to a reference voltage Vref in the feedback control circuit 130 using a voltage comparator such as the operational amplifier 132. The reference voltage Vref is a stable scaled representation of a desired regulated load voltage Vload or regulated load current Iload. The operational amplifier 132 generates an error signal 134 based on the comparison of Vsample and Vref and the magnitude of this error signal ultimately controls the operation of the transistor switch 122.
More specifically, the error signal 134 serves as a control voltage for a pulse width modulator 136 which also receives a pulse stream having a frequency f=1/T provided by an oscillator 138. In conventional DC-DC converters, exemplary frequencies f for the pulse stream include, but are not limited to, a range from approximately 50 kHz to 100 kHz. For implementations in which the load includes one or more LEDs, the light emitted from the LEDs may be perceived as being continuous as long as the switching frequency of the transistor switch 122 is greater than that capable of being detected by the human eye (e.g., greater than approximately 100 Hz). That is, an observer of light generated by the LED(s) does not perceive discrete on and off cycles (commonly referred to as the “flicker effect”), but instead the integrating function of the eye perceives essentially continuous illumination. The pulse width modulator 136 is configured to use both the pulse stream and the error signal 134 to provide an on/off control voltage signal 140 that controls the duty cycle of the transistor switch 122. In essence, a pulse of the pulse stream acts as a “trigger” to cause the pulse width modulator 136 to turn the transistor switch 122 on, and the error signal 134 determines how long the transistor switch stays on (i.e., the length of the time period ton and hence the duty cycle D).
For example, if the error signal 134 indicates that the sampled output voltage Vsample is higher than Vref (i.e., the error signal 134 has a relatively lower value), the pulse width modulator 136 is configured to provide the control signal 140 with relatively shorter duration “on” pulses or a lower duty cycle, thereby providing relatively less energy to the inductor while the transistor switch 122 is on. In contrast, if the error signal 134 indicates that Vsample is lower than Vref (i.e., the error signal has a relatively higher value), the pulse width modulator is configured to provide a control signal with relatively longer duration “on” pulses or a higher duty cycle, thereby providing relatively more energy to the inductor while the transistor switch 122 is on. Accordingly, by modulating the duration of the “on” pulses of the control signal 140 via the error signal 134, the load voltage Vload or load current Iload is regulated by the feedback control circuit 130 to approximate a desired load voltage or current represented by Vref.
In conventional buck converters such as that shown in FIG. 1, in order to change/vary one or more operating characteristics of the load (via changes to the load voltage and/or the load current), access to the feedback control circuit 130 is required to adjust the reference voltage Vref, which in turn results in a change in the regulated load current Iload (or regulated load voltage Vload, as applicable). Adjustments to Vref may be facilitated by a user interface 150 which may be an analog or digital device, such as a potentiometer or a digital-to-analog converter (DAC) used to change the reference voltage Vref. Of course, any resulting changes in Vload or Iload similarly affect all constituents of a load that may comprise multiple components; for example, in an LED-based load comprising multiple LEDs interconnected in any of a variety of serial/parallel arrangements, the operating voltage and current of each LED is affected similarly by changing conditions of the buck regulator circuit (e.g., changes to Vref).